1. Field of the Invention
Reactant conversion or product yield can often be enhanced by use of membrane reactors that operate on the principle of continuous/intermittent removal of products from the reaction zone. An important category of such reactors is that based on the use of membranes that are selective to the permeation of hydrogen. In the present invention, a device and method is described for the characterization of hydrogen-permeable membranes. This device will, in particular, find application where the permeability of hydrogen has to be measured for membranes to be used in reactors that employ electrical/electrochemical/photo-electrochemical fields that lead to generation of hydrogen.
2. Description of the Prior Art
Many reactions of importance in the process and petroleum industry are limited by thermodynamic constraints on closed system, equilibrium conversion. In such reactions, the reactant conversion can often be enhanced by use of membrane reactors that operate on the principle of continuous/intermittent removal of products from the reaction zone. A particularly important category of such reactors is that based on the use of membranes either, catalytic or non-catalytic, that are selective to the permeation of hydrogen. This configuration, besides overcoming the equilibrium conversion limitations, also provides a relatively pure stream of hydrogen that can be:                recycled to the refinery for use in hydrogenation applications; and/or        used as a clean fuel—in a fuel cell, or in direct combustion applications.        
The desire for extraction/separation of hydrogen, in its own right, has long been a goal of the petrochemical industry, as well as those interested in promoting a hydrogen-based energy economy. For example, direct decomposition of hydrogen sulfide to hydrogen and sulfur has been advocated as a valuable source for hydrogen. Conversion, however, is limited by very low reaction rates even at high temperatures. Higher conversions may, at least theoretically, be achieved through use of membrane reactors that remove hydrogen and other products from the reaction zone.
The major obstacle in the development of such systems is the availability of suitable membranes that address to satisfaction the following requirements:                Selectivity for hydrogen;        Permeability to minimize total surface area for the membranes;        Structural/mechanical strength at the operating conditions of interest; and        Economic ValueThough many membranes have been proposed, adoption at an industrial scale has been limited. For example, it is known that palladium, and its alloys, can be used as membrane material(s) for generating a very pure stream of hydrogen. However, these membranes lose their catalytic activity, permeability, as well as structural integrity in the presence of even very small quantities of sulfur-containing compounds. This limitation severely restricts application of such membranes in, say, the direct decomposition of hydrogen sulfide, among other applications.        
Some have concluded that development in membrane technology was essential for thermal decomposition of hydrogen sulfide to be adopted industrially. Mathematical models confirmed this analysis since the calculated reactor size and performance was found to depend heavily on the diffusion characteristics assumed for the hydrogen-permeable membrane, and the permeate-side flow of sweep gas flow.
In the past few years, a new class of chemical reactors has emerged in which electrical and/or photonic discharges are used to stimulate chemical reactions, for example, H2S destruction using corona discharges and microwaves. In such reactors, it should also be useful to introduce membranes that enhance conversion, or reduce power input, through removal of continuous/intermittent removal of product(s) from the reaction zone. For example, some have proposed the use of pulsed corona and silent barrier discharge reactors for the decomposition of H2S; the reactor walls, constructed from hydrogen-permeable membrane materials, remove hydrogen from the reaction zone and serve simultaneously as an electrode. High voltage pulses, with duration of about tens of nanoseconds, create an intense electric field in the reaction zone leading to the formation of non-thermal plasma. The temperature of the electrons formed from the ionization of the gaseous medium, as characterized by electron velocity/energy, is much higher than the temperature of the much larger bulk gas molecules and other ionic/charged/excited species leading to a highly efficient process. The major impediment to successful deployment of this non-thermal plasma technology is, once again, the availability of membrane materials that can handle H2S and successfully remove hydrogen from the reaction zone.
The development of membranes suitable for application in reactors based on the use electrical and/or photonic discharges to stimulate chemical reaction will likely occur. A critical requirement that must be addressed simultaneously is the development of measurement device(s) that can be used to characterize/evaluate membranes in electrical/electrochemical/photo-electrochemical fields. This is essential since hydrogen permeation mechanisms are expected vary markedly in such fields.
In conventional systems, hydrogen molecules must dissociate before they can be adsorbed on the surface of the membrane. These atoms then dissolve in the metal and diffuse, under a concentration gradient, before recombining and degassing on the permeate side of the membrane. At low temperatures, the dissociative adsorption of hydrogen can indeed be the rate-limiting step for the entire permeation process. On the other hand, hot hydrogen atoms created in the plasma region are expected to demonstrate super-permeability or plasma-driven permeation (PDP). Since energy is not required for the dissociative adsorption step, the incorporation of the hydrogen atoms into the membrane material is facilitated greatly. This phenomenon, in fact, may lead to permeabilities higher by several orders of magnitude than conventional membrane systems even at low temperatures, and for surfaces deemed conventionally unclean.
In response to the shortcomings of the art described above, the present invention is described for the characterization of hydrogen-permeable membranes. The device and method of the present invention will, in particular, find application where the permeability of hydrogen must be measured for membranes to be used in reactors that employ electrical/electrochemical/photo-electrochemical fields that lead to generation of hydrogen.